Monitoring of electric battery capacity is important in many applications, both in terms of remaining charge in a battery and in terms of storage capability of secondary batteries. By way of example, it is often important for a given task to know whether there is sufficient charge in that battery for performance of that task. Another example is the importance of knowing the point of time at which recharging of a discharging secondary battery should commence. This requires a prediction as to when a battery will be exhausted.
Alternatively or additionally, it is important to monitor the lifetime of a secondary battery or otherwise to monitor the storage capability of secondary batteries that are being charged and discharged over long periods of time.
Yet monitoring battery capacity fairly accurately has remained a difficult task, To illustrate the problem by way of example and not by way of limitation, capacity of lead acid storage batteries is specified in Ampere-Hours (AH) at a given discharge rate, typically 8 hours. For example a 64 AH battery will deliver 8 Amperes for 8 hours. This, however, is not linearly proportional. For example, that same battery will not deliver 16 Amperes for 4 hours as might have been expected from the 64 AH battery rating. Rather, at a discharge rate of 16 Amperes, that battery will be exhausted after substantially less than 4 hours. Conversely, that same battery will deliver 2 Amperes for substantially longer than 32 hours.
The problem is further complicated in that battery capacity is affected by other factors besides discharge rate. These factors include number of discharge cycles, depth of discharges, temperature and float voltage. These additional factors make capacity prediction much more difficult. A battery that has been discharged many times may behave as if it were being discharged at a certain rate when it in fact is being discharged at a very different rate. These and other known factors make it impossible to devise an algorithm for determining remaining capacity during discharge as a function of mere battery current.